One knot is one nautical mile per hour, which works out to about 1.15 mph or exactly 1.852 km/h. So knots × 1.15 = mph, and mph × 0.87 = knots. Use knots whenever you navigate with charts, because chart distances are in nautical miles and the time maths becomes simple. Use mph on inland lakes where the speed limit signs are in mph. The unit is only a display choice: 20 knots and 23 mph are the same speed.
How many mph is a knot? #
A knot is 1.15078 mph, so the quick rule is knots × 1.15 = mph. For metric, knots × 1.852 = km/h.
| Knots | MPH | km/h |
|---|---|---|
| 3 | 3.5 | 5.6 |
| 5 | 5.8 | 9.3 |
| 10 | 11.5 | 18.5 |
| 15 | 17.3 | 27.8 |
| 20 | 23.0 | 37.0 |
| 25 | 28.8 | 46.3 |
| 30 | 34.5 | 55.6 |
| 40 | 46.0 | 74.1 |
If you don’t want to do maths at the helm, “add 15%” gets you from knots to mph close enough for conversation: 20 knots is a bit over 23 mph. Going the other way, 30 mph is about 26 knots.
Why do boats use knots? #
Because the nautical mile is tied to the shape of the Earth. As NOAA puts it in its explainer on nautical miles and knots, one nautical mile equals one minute of latitude, and a knot is one nautical mile per hour. Today the international nautical mile is fixed at 1,852 metres, about 1.15 statute miles.
That link to latitude is what makes knots practical. On a nautical chart, the latitude scale on the side doubles as a distance scale: 10 minutes of latitude is 10 nautical miles. Measure a route with dividers against that scale, divide by your speed in knots, and you have the time. An 18-nautical-mile run at 12 knots takes an hour and a half. Doing the same with mph means converting every chart distance first, and each conversion is a chance to get it wrong.
The name comes from the old chip log, a wooden board on a line with knots tied at regular intervals. Sailors threw it overboard and counted how many knots ran out in a set time. The unit stayed after the method disappeared.
Should I use knots or mph on my boat? #
| Where you boat | Best unit | Why |
|---|---|---|
| Coastal and offshore | Knots | Charts, tide tables, forecasts and other boats all use knots and nautical miles |
| Sailing and racing | Knots | Wind speed, polars and race instructions use knots |
| Inland lake with posted mph limits | MPH | Matches the signs and what you’re used to in a car |
| Rivers and canals with km/h limits (common in Europe) | km/h | Matches local rules |
| Towing skiers or wakeboarders | MPH or km/h | Rider and tow-speed guidance is usually given in land units |
Whichever you choose, use one unit for everything on board: speed, distance and the speed limit alarm. Mixing mph speeds with nautical-mile distances is how fuel and time estimates go wrong.
Does the unit change GPS accuracy? #
No. A GPS reading of 20 knots and 23 mph is the same measurement shown two ways. What does matter on the water is what GPS measures: speed over ground, not speed through the water. In a current, a GPS and a paddle wheel or pitot gauge will disagree whatever units they show. Speed over ground vs speed through water explains the gap and how to use it.
How to set up a phone as a boat speedometer in knots #
A phone makes a good second speedometer on a boat that has one, and a primary one on a dinghy, kayak, jet ski or small outboard that doesn’t. With Speedometer GPS:
- Switch the unit to knots. Tap the unit button until it shows knots, or choose it in Settings. Trip distance switches to nautical miles (distances under 0.1 nautical miles show in metres), and the app remembers your choice.
- Check the heading line. Under the readout you get your course (N, NE and so on, plus degrees) and altitude. The course comes from GPS while you’re moving, so it’s the direction you’re actually travelling, which in a cross current or crosswind can differ from where the bow points.
- Set a speed limit alarm if you need one. The knot presets are 5, 8, 10, 20 and 30, adjustable in 1-knot steps. Pass the limit and the readout turns red and the phone buzzes and beeps, repeating every 10 seconds until you slow down.
- Press Start when you leave the dock and Save at the end. The trip keeps distance, top speed, average speed while moving, the route and a speed chart, and you can export it as GPX.
One limit to know: to keep GPS drift out of trips, the app ignores steps of less than 5 metres between one-second fixes, so below about 9.7 knots (paddling, trolling, sailing in light air) its trip distance under-counts. The speed reading isn’t affected. Boat trip distance in nautical miles covers the workarounds.
The app works offline, since GPS needs no mobile data, which is what you want a few miles out. It’s free and ad-supported, with a one-time purchase to remove the ads.
Practical tips for phones on boats #
- Use a waterproof case and a lanyard or a secure mount. Most phones sink.
- Keep it out of direct sun where you can. Phones dim the screen or slow charging when they overheat.
- Plug it in on long runs. GPS once a second plus an always-on screen uses real power.
- Don’t make it your only navigation. A phone is a good speedometer and trip log, not a replacement for charts, a compass or a chartplotter.
Knots in speed zones and slow areas #
Many no-wake areas don’t post a number at all: “slow, no wake” means slow enough that you don’t make a wake, whatever the speedometer says. Where a numeric limit is posted, set your display to the same unit as the sign, and set the alarm a little under the limit, since GPS speed over ground can differ from your speed through the water in a current. For more on alarms, see how GPS speed alerts work, and for switching between land units, mph to km/h on a GPS speedometer.
Frequently asked questions #
How many mph is 1 knot? #
About 1.15 mph (1.15078 to be exact). One knot is also exactly 1.852 km/h, because the international nautical mile is defined as 1,852 metres.
Why is a knot called a knot? #
It comes from the chip log, a board on a knotted line that sailors threw overboard. Counting the knots that ran out in a set time gave the ship’s speed, and the name outlived the method.
Is GPS speed in knots accurate on a boat? #
On open water with a clear sky, a phone’s GPS speed is typically within a few tenths of a knot at a steady speed. It’s speed over the ground, so in a current it will differ from a paddle wheel or pitot that measures speed through the water.
Should I use knots on a lake? #
You can, but mph or km/h is often easier inland, because lake speed limits and tow-sport guidance are usually posted in land units. Use knots if you navigate with charts or also boat on the coast.
What is the difference between a nautical mile and a mile? #
A statute mile is 5,280 feet (1,609 m). A nautical mile is 1,852 m, about 6,076 feet or 1.15 statute miles. The nautical mile is based on one minute of latitude, which is why it’s used with charts.